Influenza can infect as much as 5-15% of the world population, resulting in 3-5 million cases of severe illness and up to 500,000 deaths each year. In the U.S. alone, flu epidemics lead to approximately 300,000 influenza-related hospital admissions and 36,000 influenza-related deaths annually in addition to an estimated cost of $12 billion per year (Poland 2001; Simonsen et al. 2007, PMID: 17897608). Current seasonal influenza vaccines are produced with strains recommended by the World Health Organization about 9-12 months ahead of the targeted season (Carrat et al. 2007). The vaccines typically contain two type A influenza strains and one type B strain, which are predicted to be the most likely strains to cause the upcoming flu epidemic.
However, there are inherent disadvantages associated with the preparation of conventional influenza vaccines such as the uncertainty of the actual circulating strain, the need for annual updating of the manufacturing process and preparation of reagents for vaccine lot release. Furthermore, mismatches between the strains selected for vaccine preparation and the circulating viruses were found to be responsible for much reduced efficacy of the seasonal influenza vaccines (Bridges et al. 2000; De Filette et al. 2005). Clearly, the drawbacks associated with traditional vaccine preparation would be drastically exacerbated in the event of an outbreak of pandemic influenza, given a perceivably much shortened timeframe available for the production of prophylactic vaccines for global needs. All these problems concerning the influenza vaccines are largely due to one single biological property of the influenza virus itself, i.e. the constant mutations of the virus surface proteins hemagglutinin (HA) and neuraminidase (NA).
Currently, influenza A viruses representing 16 HA and 9 NA subtypes have been detected in wild birds and poultry throughout the world (Zambon 1999; Treanor 2004; Fouchier 2005). Frequent antigenic drifting or shifting of HA and NA prompted numerous exploratory investigations of vaccines that are intended to induce host immune responses against viral proteins that are less subject to antigenic fluctuations. Of these conserved antigenic determinants, the nucleoproteins (NP) and Matrix (M) have been shown to induce protective immunity against diverse strains of the viruses (Frace et al. 1999; Epstein et al. 2002; de Filette 2005; Mozdzanowska et al. 2003; Fan et al. 2004).
Furthermore, it has been suggested that cell-mediated immune response rather than humoral immune responses protect the animals immunized with NP-based vaccines while antibody-mediated protections against lethal challenges of various subtypes of influenza virus were reported with the use of M2-based vaccines (Neirynck et al. 1999; de Filette et al 2005; Mozdzanowska et al. 2003). None of these universal vaccines appears to prevent viral infection in animal studies although prevention of clinical diseases was found to be promising (Gerhard et al. 2006).
Given the importance of neutralizing antibodies against NA in preventing influenza infection, the conserved regions in the NA proteins have also received great attention in recent years.
There remains a need in the art for reagents that may be universally used to detect influenza viruses or proteins therein, especially neuraminidase (NA) proteins.